1. Field of the Invention
This invention relates to switchgear for electric power distribution systems. More particularly, it relates to the construction and support of the rigid electrical conductors forming buses which route the power within the switchgear cabinet, and especially to the vertical buses or risers which distribute power to multiple electrical apparatus stacked in the switchgear cabinet.
2. Background Information
Switchgear assemblies are combinations of electrical apparatus for electric power distribution systems mounted in metal cabinets. The electrical apparatus typically includes switching devices such as circuit breakers and network protectors which provide protection as well as switching, disconnect switches which isolate or separate parts of the distribution system, and transfer switches which are used to connect the system to alternative power sources. A switchgear assembly can also include instrumentation such as metering equipment.
Typically, several pieces of electrical apparatus are mounted in a single cabinet, usually stacked vertically. Multi-phase input and output electrical conductors must be connected to each piece of electrical apparatus. In a common arrangement, a set of multi-phase rigid conductors or bus bars extend transversely through the cabinet to form a cross bus. A set of multi-phase risers extending vertically within the cabinet behind the electrical apparatus connects the cross bus conductors with the individual electrical apparatus through stabs which are engaged by quick disconnects on back faces of the electrical apparatus as the apparatus is fully inserted into the cabinet. Another set of rigid conductors, called runbacks, engage other quick disconnects on the electrical apparatus and extend rearward between the risers for connection to cabling extending out of the cabinet.
Historically, the vertical bus assemblies or risers in switchgear have utilized flat copper conductors. Currently, there is a riser configuration which utilizes two U-shaped conductors mounted back to back to form an H configuration. This geometry with its increased section modulus, provides a more rigid bus which exhibits improved performance against the magnetic forces incurred during short circuit conditions. For higher ampere ratings, additional thick flat bars are added along one or both sides of the H. These combinations of conductors utilized to achieve various current ratings are not the most efficient constructions. The "skin" effect phenomena (ratio of ac to dc resistance) in conductors dictates that the bulk of the ac current flows in the outer perimeter of a current path. In effect, material internal to the outer perimeter of a current path is significantly less efficient than the material on the perimeter. The H shaped construction results in substantial material internal to the perimeter (the cross bar of the H). In addition, the higher rated risers with the additional flat conductors on the outside make the vertical legs of the H internal. Another consideration in the construction of the vertical bus or risers within switchgear is the interface of these buses with the stabs which connect the risers to the individual pieces of electrical apparatus.
There is a need for improvement in switchgear and particularly in the construction and mounting of the vertical bus bars or risers.
There is a particular need for providing an efficient construction of the risers for carrying current which can also resist the high magnetic forces experienced during short circuit conditions.
There is a further need for such improved switchgear risers which can accommodate for various current ratings with minimum modifications and with adequate resistance to the magnetic repulsion forces.
There is still another need for such an improved vertical bus bar arrangement in which the risers can be easily connected to the stabs.